1. Field of the Invention
The present invention relates generally to a communication system using a variable transmission scheme, and in particular, to an apparatus and method for determining an optimum modulation scheme and code rate in a given environment.
2. Description of the Related Art
Although a typical communication system adopts a fixed transmission scheme for data transmission, communication technology has been developed to transmit data in a variable transmission scheme. For example, a future generation mobile communication system developed by the 3GPP2 (the 3rd Generation Partnership Project 2) adopts a variable transmission scheme.
Thee L3QS (Lucent technology co., LSI co., LG electronics and Qualcomm co., Samsung electronics co.) framework proposal contributed to the 3GPP2 TSG-C WG5 in June 2000 for standardization of 1× EV-DV (1× Evolution-Data and Voice) developed by the 3GPP2 has provided a synopsis of data rates with respect to six EP (Encoder Packet) sizes (i.e., 384, 768, 1536, 2304, 3072, and 3840 bits) in six tables. These data rate tables are designed to increase channel transmission efficiency by supporting a variety of packets transmitted at each instant. An EP is a packet input to an encoder. It is necessary to select an appropriate EP size among the six EP sizes according to channel condition, data backlog (i.e., the status of buffers having data received from a higher layer), and the number of available Walsh codes (NAWC). One of the significant factors for EP size selection is selection of a modulation scheme. For packets with the same EP size, different modulation schemes and different code rates of an error correction code may be applied depending on their transmission durations and NAWC.
Each of the data rate tables lists 69 to 111 different modulation and code rate combinations according to the number of available Walsh codes (NAWC) and the number of slots per sub-packet. For example, if EP size=384, NAWC=28, and NOS (Number Of Slots)=8, then a modulation order=2 (QPSK) and a code rate=0.017857. If EP size=384, NAWC=27, and NOS=8, then a modulation order=2 (QPSK) and a code rate=0.037037. Therefore, it is very important to select an optimum modulation scheme and code rate in a given environment in a communication system using a variable transmission scheme in time.